"Humic substances (HS) are complex and heterogeneous mixtures of polymers formed in soils, sediments and natural waters during the decay of plant biomass. HS represent an important carbon source in aquatic environments accounting for up to 80% of the total dissolved organic carbon (DOC). Historically, HS were considered to be biologically inert but discovery of the “microbial loop” changed this view in which DOC enters planktonic food webs through incorporation into bacterial biomass and protozoan grazing upon bacteria.Despite extensive research it is still not clear how exactly HS are converted and degraded by microorganisms in nature. We particularly lack a detailed understanding of the metabolic diversity present in a community that is used to attack the complex structure of these heterogeneous polymers. Based on current structural concepts it is assumed that HS cannot be degraded by simple enzymatic reactions but rather by a combination of different enzymes including peroxidases, phenol oxidases and many carbohydrate active enzymes.The proposed project aims to resolve the genomic and metabolic diversity of freshwater microbial communities involved in HS cycling using state of the art sequencing-based approaches. Metagenomic and metatranscriptomic approaches will be used to perform in-depth genomic analysis of freshwater microbial communities and to study their physiological response to HS based on gene-expression analysis. Different habitats and ecological niches will be sampled and analyzed to enable detailed comparative analysis (e.g. aerobic vs. anaerobic habitats).Considering the environmental and economic importance of HS it is very important to understand mechanisms involved in HS cycling in nature. The proposed project will provide the scientific community with a detailed description of microbial diversity and activity involved in freshwater HS cycling and the data will be a valuable foundation for future research in this field."